In the coating of photographic layers on a support such as a film base or paper, a plurality of individual layers are often coated on the support simultaneously, with each successive layer being superimposed on the layer below by means of a coating hopper. One type of coating hopper, known as a multiple slide hopper, is comprised of individual slide elements which are separated by slots and cavities. By introducing each coating liquid into a cavity, the liquid stream is distributed to the desired width and then metered uniformly across the coating width by flowing through the narrow slot. Upon exiting the slot, the layer flows by gravity down the inclined slide surface. Layers of coating liquids then become superimposed on one another as layers from upstream slots flow over the layers exiting from the downstream slots. At the end of the slide surface, the liquid flows onto and coats the moving web.
A problem occurs when the narrow hopper slots become partially blocked by solid particles, gel slugs or air bubbles or when the slot surface has nicks or scratches. The flow in the vicinity of the blockage or scratch is disturbed and becomes three-dimensional. If this disturbance does not heal and the flow become two-dimensional again downstream, a deficit or longitudinal depression will develop in the layer that was delivered through the slot which contained the blockage or scratch. As the upper layers become superimposed on the layer containing the depression, they will fill in the deficit as gravity and surface tension forces act to level the top surface of the liquid layers on the hopper slide. As a result, a variation in thickness of one or several layers will occur across the width of the layers. This thickness variation of the coated layers, which is readily visible in layers having substantial optical density, creates a longitudinal streak. If sufficiently severe, the longitudinal streak destroys the value of all or part of the coated product. In the coating of photographic films and papers such streaks and lines can be a major source of waste and can add to the manufacturing costs.
The patent to Hughes, U.S. Pat. No. 3,508,947, mentions the possibility of coating a plurality of layers of the same composition in a curtain coating process (see col. 14, lines 10-23) and says that the formation of "slot lines" as occur with a single-slot extrusion-type hopper is avoided by this means. There is no suggestion, however, of any precise manner for predetermining the variables of the coating process to reduce such coating defects while achieving high coating rates. Moreover, there is no suggestion of the serious problem of streaks caused by blockages in the hopper slots. A need exists, therefore, for a method for controlling the coating procedure to reduce the severity of longitudinal streaks in the coated layers which are caused by slot blockages. The need also exists for a method for achieving high coating rates without forming severe streaks. The present invention provides such a method.